Adolescent girls, particularly African American girls, are at increased risk of adopting a sedentary lifestyle, increasing their likelihood of developing cardiovascular diseases. There have been few school-based physical activity interventions for adolescents, even though this is a time in which their physical activity levels dramatically decline. Long-term follow-up of intervention effectiveness is virtually non-existent. The proposed project is a randomized, controlled trial of a life skills-oriented, comprehensive aerobic physical activity intervention (CAP) conducted during physical education (PE) class versus standard PE to increase physical activity and cardiorespiratory fitness in predominantly African American adolescent girls. Long-term follow-up of intervention effectiveness over 2 1/2 years will be assessed. The intervention will be conducted over two semesters and will incorporate behavioral skills training, including strong generalization and relapse prevention training components, into a PE class that focuses on aerobic exercise. A unique feature of the intervention is a family support component operationalized by a parent support training session and mail-delivered support tips and reminders to family members. Follow-up measures will be assessed through 11th grade (over 2 1/2 years) to determine long- term effectiveness of the interventions on physical activity level, cardiorespiratory fitness, and other selected cardiovascular risk factors. Ninth grade girls (3 cohorts in 3 successive years; n=300) attending an all-girls public high school in Baltimore, MD will be randomly assigned to the CAP or to standard PE. Physical activity level will be determined by the 7-day physical activity recall and by objective heart rate/motion sensor monitoring in a 50 percent random subsample. Cardiorespiratory fitness will be estimated from a submaximal step test. Selected cardiovascular risk factors (blood pressure, body composition, lipids) will also be determined. Follow-up will be assessed at 5, 11, 14, and 30 months post-randomization, providing long-term assessment of physical activity level and cardiorespiratory fitness compared with standard PE. This project will determine the ability of comprehensive interventions delivered in a school setting to reduce the rate of decline in physical activity typically observed in adolescent girls.